A secondary battery such as a nickel-cadmium battery, nickel-hydrogen battery and lithium ion battery is widely used in electronic devices or components, particularly in cellular phones, notebook personal computers, video cameras, satellites and electric or hybrid cars. Conventionally, in the case of a secondary battery using a strong alkali electrolyte, such as nickel-cadmium and nickel-hydrogen, a casing composed of a nickel-plated cold-rolled steel sheet or a plastic casing has been used. In a battery using a nonaqueous electrolyte, such as a lithium ion battery, the electrolyte incorporated into an aluminum pouch is wrapped with a plastic casing or a casing composed of a nickel-plated steel sheet or stainless steel sheet.
In recent years, with the miniaturization of electronic and electric components, a secondary battery is also required to be compact and lightweight. In such a trend, thinning of a secondary battery container is attracting attention as a tool allowing for mounting of a larger amount of electrolyte or ion in a limited volume and thereby enabling an increase in battery capacity. However, if the strength of the container is decreased due to thinning, a risk of liquid leakage of the electrolytic solution due to deformation or breakage when an outer force or piercing is applied. The liquid leakage of the electrolytic solution has a high possibility of causing serious damage to the apparatus where the secondary battery is incorporated. If the container member is plastic or aluminum, strength is insufficient with a wall thickness of 200 μm or less. For realizing more thinning, a material having high strength is necessary. Considering mass production, the material is preferably a general-purpose material.
The material satisfying such required properties includes a stainless steel foil. Stainless steel foil is a foil obtained by thinning a stainless steel to a thickness of 200 μm or less. Stainless steel generally has high strength such that the tensile strength or Vickers hardness is 2 to 10 times that of plastic or aluminum, and is promising as a thinned material for the secondary battery container.
However, the metal foil has poor corrosion resistance in an electrolytic solution and when used for a battery casing or a lead wire, is sometimes eroded by the electrolytic solution. As a countermeasure to make up for the weakness in corrosion resistance of the metal foil, a metal foil comprising a chromate-treated aluminum foil having stacked thereon an acid-modified polyolefin film having barrier property against a corrosion-causing substance is disclosed (see, for example, Japanese Unexamined Patent-Publication (Kokai) No. 2000-357494). By this technique, the workability or corrosion resistance can be improved to a certain extent. However, the film sometimes fails to exert sufficiently high adherence when the thickness of the chromate treatment layer is inadequate. Also, if the chromate treatment layer is thicker than necessary, this is disadvantageous in that a chromate having a large environmental load is uselessly used. In particular, a stainless steel foil has a problem that adherence between the chromate treatment layer and the stainless steel of the base material is weak, and, for example, the chromate treatment layer comes off from the base material to cause separation of the film.
A stainless steel foil obtained by drawing a stainless steel to a thickness of 200 μm or less is attracting attention as a material having a thin wall thickness and excellent mechanical strength and is being widely applied as a material for home electric components such as refrigerator or heat-insulated pouch, OA equipment components such as arm, load beam, flexure and cover of a hard disc drive, or lead wires or pouches of a secondary battery. However, stainless steel foil generally has a problem in workability, and a technique or the like for optimizing the composition and heat treatment to improve the workability is known (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2004-52100). A lubricant is necessary at the shaping such as drawing in many cases, and three steps of degreasing, coating of lubricating oil and removal of lubricating oil are required before and after the working. These steps not only lead to an increase in the step at the shaping but also sometimes require a separation step of separating cleaning water and lubricating oil. Even when a lubricant is used, there is a case where the mold cannot be satisfactorily prevented from abrasion and the mold life cannot be sufficiently prolonged.
Furthermore, as described above, a secondary battery has many uses in a mobile product, and since the battery of a mobile product can be replaced by an individual user, the need for good design property is high. However, it is technically cumbersome and involves a relatively high cost to clearly print letters directly on a stainless steel foil by printing or provide color variations. Thus, a high design property is difficult to impart to the stainless steel foil.
The present invention has been made by taking into consideration these problems and an object of the present invention is to provide a stainless steel foil capable of maintaining strong adherence to a film even in an electrolytic solution to exhibit good corrosion resistance and excellent in workability, design property and piecing resistance, a container using the stainless steel foil, and a secondary battery using the container.